Spin-wave excitations in multiferroic heterostructures and CoFeB/YIG bilayers

Sampo Hämäläinen

Spin-wave excitations in multiferroic heterostructures and CoFeB/YIG bilayers

Julkaisun otsikon käännös: Spin-aaltoviritykset moniferroisissa heterorakenteissa ja CoFeB/YIG kaksikerrosrakenteissa

Sampo Hämäläinen

Tutkimustuotos: Doctoral Thesis › Collection of Articles

Abstrakti

Spin-aallot ovat kollektiivisia viritystiloja magneettisissa materiaaleissa. Näillä aalloilla voi olla pitkä koherenssipituus, korkea ryhmänopeus ja aallonpituus nanometriluokassa. Aaltoluonnetta voidaan käyttää energiatehokkaassa laskennassa ja muissa laitteissa kuten mikroaaltosuodattimissa ja muistielementeissä. Koska spin-aaltojen eteneminen ei vaadi elektronien liikettä, Joule-häviöt voidaan välttää. Tässä väitöskirjassa esitän tuloksia spin-aaltojen luomisesta venymäkytketyissä moniferroisissa rakenteissa ja YIG/CoFeB kaksikerroskalvoissa. Moniferroinen materiaali perustuu BaTiO3 aihiosta, jossa on säännöllinen ferroelastinen raitadomainrakenne, sekä ferromagneettisesta kalvosta. Venymäkytkeymä aihion rajapinnalla aiheuttaa anisotropian muodostumisen ferromagneettisen kalvoon käänteisen magnetostriktion kautta, jolloin anisotropia-akselit seuraavat täydellisesti aihion domainrakennetta ja domainseinämät jäätyvät rajapintaan. Jäätynyttä seinämää voidaan käyttää lyhyiden spin-aaltojen luomiseen ajamalla vaihtovirtaa seinämän läpi. Sähkövirta aiheuttaa seinämän edestakaisen liikehdinnän, joka säteilee spin-aaltoja käytetyllä ajotaajuudella. Jos sama moniferroinen rakenne on saturoituneessa tilassa, seisovia spin-aaltoja muodostuu anisotropiaraitarakenteisiin sekä eteneviä spin-aaltoja säteilee rajapinnasta. Tässä tapauksessa, anisotropia-akselin kääntyminen suhteessa magnetisaatioon aiheuttaa säännöllisen ja nopean muutoksen efektiivisessä magneettikentässä. Spin-aaltojen säteileminen johtuu naapurialueiden erilaisista pakotetuista oskillaatioamplitudeista. Sama mekanismi synnyttää seisovia spin-aaltoja YIG/CoFeB kaksikerrosrakenteissa, joissa dynaaminen vaihtovuorovaikutus kalvojen rajapinnalla säteilee aaltoja YIG-kalvoon. Moniferroista materiaalia käytetään myös tutkimaan 90o domainseinämän vaikutusta spin-aaltojen etenemiseen. Seinämän leveyttä voidaan determinisesti muokata kapean ja leveän välillä kääntämällä magnentisaation suunta kärki-perä asetelmasta kärki-kärki asentoon. Brillouin sirontakokeilla ja mikromagneettisilla simulaatioilla näytetään, että spin-aaltojen transmissio muuttuu nollasta lähes 100% kun seinämän tilaa muutetaan. Tätä suurta muutosta voitaisiin käyttää esimerkiksi spin-aalto logiikassa tai stabiileina muistielementteinä.

Julkaisun otsikon käännös	Spin-aaltoviritykset moniferroisissa heterorakenteissa ja CoFeB/YIG kaksikerrosrakenteissa
Alkuperäiskieli	Englanti
Pätevyys	Tohtorintutkinto
Myöntävä instituutio	Aalto-yliopisto
Valvoja/neuvonantaja	van Dijken, Sebastiaan, Vastuuprofessori
Kustantaja	Aalto University
Painoksen ISBN	978-952-60-8204-2
Sähköinen ISBN	978-952-60-8205-9
Tila	Julkaistu - 2018
OKM-julkaisutyyppi	G5 Tohtorinväitöskirja (artikkeli)

Tutkimusalat

spin aallot
moniferroiset heterorakenteet
magnoniikka

Pääsy asiakirjaan

http://urn.fi/URN:ISBN:978-952-60-8205-9Lisenssi: Määrittelemätön

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@phdthesis{6ce824a32e2644e38d24b0f6ed2e85fe,

title = "Spin-wave excitations in multiferroic heterostructures and CoFeB/YIG bilayers",

abstract = "Spin waves are collective excitations in magnetic materials that can exhibit long coherence lengths, high group velocities, and wavelengths down to the nanometer scale. The properties of spin waves could be exploited in low-power wave-like computing and in other devices such as microwave filters and data storage elements. In contrast to the motion of electrons, spin waves propagate via transmission of angular momentum. Power dissipation by Joule heating can thus be avoided. In this thesis, I present results on spin-wave excitation and confinement in strain-coupled multiferroic heterostructures and YIG/CoFeB bilayers. The multiferroic heterostructures consist of a ferroelectric BaTiO3 substrate with a regular pattern of ferroelastic stripe domains and a ferromagnetic film. Strain coupling at the interface imprints an anisotropy pattern in the ferromagnetic film via inverse magnetostriction. As a result, the domain pattern of the BaTiO3 substrate is fully transferred to the ferromagnetic film and magnetic domain walls are firmly pinned by the induced anisotropy boundaries. The pinned magnetic domain walls can be used to excite short-wavelength spin waves using an ac electric current. In this case, the spin-polarized current drives the domain wall into oscillatory motion, causing spin-wave emission at the same frequency as the actuation signal. If the same multiferroic system is saturated by an external magnetic field and excited by a uniform microwave field, standing spin waves form within the domains and propagating spin wave are emitted from the anisotropy boundaries. Here, rotations of the anisotropy axis with respect to the direction of magnetization create a regular modulation of the effective field. The launching of spin waves from a single anisotropy boundary is explained by coupling between two forced magnetization precessions in neighboring stripe domains. A similar mechanism is also responsible for the excitation of perpendicular standing spin waves in exchange-coupled YIG/CoFeB bilayers. In this system, forced precession in YIG and CoFeB generate a dynamic exchange torque at the interface. Lastly, multiferroic heterostructures are used to study spin wave transmission through narrow and wide 90° domain walls. Deterministic switching between these wall types is realized by realigning the magnetization from a head-to-tail (narrow wall) to a head-to-head (broad wall) configuration. Brillouin light scattering experiments and micromagnetic simulations demonstrate that this reversible switching effect toggles the transmission of spin waves from nearly 0% to 100%. Reconfigurable modulations of spin-wave transmission could be used in logic devices or non-volatile memory cells.",

keywords = "spin waves, multiferroic heterostructures, magnonics, spin aallot, moniferroiset heterorakenteet, magnoniikka, spin waves, multiferroic heterostructures, magnonics",

author = "Sampo H{\"a}m{\"a}l{\"a}inen",

year = "2018",

language = "English",

isbn = "978-952-60-8204-2",

series = "Aalto University publication series DOCTORAL DISSERTATIONS",

publisher = "Aalto University",

number = "187",

address = "Finland",

school = "Aalto University",

}

TY - THES

T1 - Spin-wave excitations in multiferroic heterostructures and CoFeB/YIG bilayers

AU - Hämäläinen, Sampo

PY - 2018

Y1 - 2018

N2 - Spin waves are collective excitations in magnetic materials that can exhibit long coherence lengths, high group velocities, and wavelengths down to the nanometer scale. The properties of spin waves could be exploited in low-power wave-like computing and in other devices such as microwave filters and data storage elements. In contrast to the motion of electrons, spin waves propagate via transmission of angular momentum. Power dissipation by Joule heating can thus be avoided. In this thesis, I present results on spin-wave excitation and confinement in strain-coupled multiferroic heterostructures and YIG/CoFeB bilayers. The multiferroic heterostructures consist of a ferroelectric BaTiO3 substrate with a regular pattern of ferroelastic stripe domains and a ferromagnetic film. Strain coupling at the interface imprints an anisotropy pattern in the ferromagnetic film via inverse magnetostriction. As a result, the domain pattern of the BaTiO3 substrate is fully transferred to the ferromagnetic film and magnetic domain walls are firmly pinned by the induced anisotropy boundaries. The pinned magnetic domain walls can be used to excite short-wavelength spin waves using an ac electric current. In this case, the spin-polarized current drives the domain wall into oscillatory motion, causing spin-wave emission at the same frequency as the actuation signal. If the same multiferroic system is saturated by an external magnetic field and excited by a uniform microwave field, standing spin waves form within the domains and propagating spin wave are emitted from the anisotropy boundaries. Here, rotations of the anisotropy axis with respect to the direction of magnetization create a regular modulation of the effective field. The launching of spin waves from a single anisotropy boundary is explained by coupling between two forced magnetization precessions in neighboring stripe domains. A similar mechanism is also responsible for the excitation of perpendicular standing spin waves in exchange-coupled YIG/CoFeB bilayers. In this system, forced precession in YIG and CoFeB generate a dynamic exchange torque at the interface. Lastly, multiferroic heterostructures are used to study spin wave transmission through narrow and wide 90° domain walls. Deterministic switching between these wall types is realized by realigning the magnetization from a head-to-tail (narrow wall) to a head-to-head (broad wall) configuration. Brillouin light scattering experiments and micromagnetic simulations demonstrate that this reversible switching effect toggles the transmission of spin waves from nearly 0% to 100%. Reconfigurable modulations of spin-wave transmission could be used in logic devices or non-volatile memory cells.

AB - Spin waves are collective excitations in magnetic materials that can exhibit long coherence lengths, high group velocities, and wavelengths down to the nanometer scale. The properties of spin waves could be exploited in low-power wave-like computing and in other devices such as microwave filters and data storage elements. In contrast to the motion of electrons, spin waves propagate via transmission of angular momentum. Power dissipation by Joule heating can thus be avoided. In this thesis, I present results on spin-wave excitation and confinement in strain-coupled multiferroic heterostructures and YIG/CoFeB bilayers. The multiferroic heterostructures consist of a ferroelectric BaTiO3 substrate with a regular pattern of ferroelastic stripe domains and a ferromagnetic film. Strain coupling at the interface imprints an anisotropy pattern in the ferromagnetic film via inverse magnetostriction. As a result, the domain pattern of the BaTiO3 substrate is fully transferred to the ferromagnetic film and magnetic domain walls are firmly pinned by the induced anisotropy boundaries. The pinned magnetic domain walls can be used to excite short-wavelength spin waves using an ac electric current. In this case, the spin-polarized current drives the domain wall into oscillatory motion, causing spin-wave emission at the same frequency as the actuation signal. If the same multiferroic system is saturated by an external magnetic field and excited by a uniform microwave field, standing spin waves form within the domains and propagating spin wave are emitted from the anisotropy boundaries. Here, rotations of the anisotropy axis with respect to the direction of magnetization create a regular modulation of the effective field. The launching of spin waves from a single anisotropy boundary is explained by coupling between two forced magnetization precessions in neighboring stripe domains. A similar mechanism is also responsible for the excitation of perpendicular standing spin waves in exchange-coupled YIG/CoFeB bilayers. In this system, forced precession in YIG and CoFeB generate a dynamic exchange torque at the interface. Lastly, multiferroic heterostructures are used to study spin wave transmission through narrow and wide 90° domain walls. Deterministic switching between these wall types is realized by realigning the magnetization from a head-to-tail (narrow wall) to a head-to-head (broad wall) configuration. Brillouin light scattering experiments and micromagnetic simulations demonstrate that this reversible switching effect toggles the transmission of spin waves from nearly 0% to 100%. Reconfigurable modulations of spin-wave transmission could be used in logic devices or non-volatile memory cells.

KW - spin waves

KW - multiferroic heterostructures

KW - magnonics

KW - spin aallot

KW - moniferroiset heterorakenteet

KW - magnoniikka

KW - spin waves

KW - multiferroic heterostructures

KW - magnonics

M3 - Doctoral Thesis

SN - 978-952-60-8204-2

T3 - Aalto University publication series DOCTORAL DISSERTATIONS

PB - Aalto University

ER -

Spin-wave excitations in multiferroic heterostructures and CoFeB/YIG bilayers

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